In the News (Mon 17 Dec 18)

The observation of these changes in Mercury’sperihelion was one of the first confirmations of Einstein’s general theory of relativity, which predicted such variation due to the curvature of space caused by the enormous mass of the Sun.

Mercury'sprecession showed the effects of mass dilation, providing a crucial observational confirmation of one of Einstein's theories—Mercury is slightly heavier at perihelion than it is at aphelion because it is moving faster, and so it slightly "overshoots" the perihelion position predicted by Newtonian gravity.

Mercury’smeandensity, 5.4 g/cm³, is nearly as great as that of Earth and is higher than that of any of the other planets.

The point in Mercury'sorbit at which the planet is closest to the Sun (called the planet’sperihelion) moves a tiny amount every orbit, too much to be accounted for by the gravitational influence of other planets.

Mercury is a poor reflector of sunlight because its surface consists of rough, porous, dark-colored rock.

The innermost planet of the solar system and second smallest after Pluto; it is surpassed in size (but not in mass) by both Ganymede and Titan.

Mercury'sperihelionprecesses around the Sun at a rate that 19th century astronomers couldn't properly explain.

Mercury is the second densest major body in the solar system, after the Earth, pointing to a relatively enormous iron core with a radius of 1,800 to 1,900 km –; proportionately much larger than Earth's and making up most of the planet.

Mercury is the smallest of the inner, rocky planets (Mars, Earth, and Venus) and the closest to the Sun.

This hypothesis also predicts that Mercury should have higher concentrations of elements that have very high boiling points (called refractory elements), such as calcium and aluminum, and not much of the elements that boil at low temperatures (called volatile elements), such as sodium and potassium.

A key feature of lava flows on many planets is a composition different from the surface the lava sits on, as shown by the maria on the Moon being darker and higher in FeO.

Mercury'sprecession showed the effects of mass dilation, providing a crucial observational confirmation of one of Einstein's theories - Mercury is slightly heavier at perihelion than it is at aphelion because it is moving faster, and so it slightly "overshoots" the perihelion position predicted by Newtonian gravity.

Unfortunately, the same face of the planet was lit at each of Mariner 10's close approaches, resulting in less than 45% of the planet's surface being mapped.The spacecraft made three close approaches to Mercury, the closest of which took it to within 327 km of the surface.

Among these, Mercury is an extreme: the smallest, the densest (after correcting for self-compression), the one with the oldest surface, the one with the largest daily variations in surface temperature - and the least explored.

Mercury's uncompressed density (what its density would be without compaction of its interior by the planet's own gravity) is 5.3 grams per cubic centimeter, by far the highest of all the terrestrial planets.

Mercury's axis of rotation is oriented nearly perpendicular to the planet'sorbit, so that in the polar regions sunlight strikes the surface at a constant grazing angle.

The small and rocky planetMercury is the closest planet to the Sun; it speeds around the Sun in a wildly elliptical (non-circular) orbit that takes it as close as 47 million km and as far as 70 million km from the Sun.

Mercury completes a trip around the Sun every 88 days, speeding through space at nearly 50 km per second, faster than any other planet.

Because Mercury is so close to the Sun, it is hard to see from Earth except during twilight.